Before pulling any more hair, you might want to fix a small bug:
includes/modules/order_total/ot_group_pricing.php
line 21 has:
PHP Code:
$this->credit_tax = MODULE_ORDER_TOTAL_GROUP_PRICING_CREDIT_TAX;
change it to:
PHP Code:
$this->tax_class = MODULE_ORDER_TOTAL_GROUP_PRICING_TAX_CLASS;
As an addendum, the "include shipping" and "include tax" settings, if set to "true", work like this:
"Order Total" minus "Shipping Costs" minus "Tax Amount" = "Amount to apply discount to"
ie: $50.00 - $3.50 - $2.80 = $43.70 .... if discount rate is 10%, then the discount will be $4.37
Hope that helps clarify some of how it works.
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